The present invention relates to a multielement rare earth-iron interstitial hard magnetic material having a ThMn12 type crystal structure. The present invention further relates to processes for producing isotropic and anisotropic magnetic powder, and for producing isotropic and anisotropic magnet.
Currently, the rare earth-iron based material used for producing hard magnet is Nd2Fe14B, in which the process used for producing Nd2Fe14B type bonded magnet is melt spinning or HDDR technique. However, the magnetic powder obtained by using these processes is generally isotropic, with the maximum energy product of 60-110 KJ/m3(8-13 MGOe). It is an anisotropic magnetic powder having a high magnetic energy product that is sought to be developed. The Nd2Fe14B type magnet has a low Curie temperature, and is insufficient in anti-oxidation capacity. Furthermore, since spin reorientation occurs at the temperature around 130K, and easy magnetization direction deviates from C axis, the permanent magnetic properties vanish at low temperatures. Iriyama Kyohiko et al. and J. M. D. Coey et al. teach a R2Fe17Nx based rare earth-iron-nitrogen permanent magnetic material (Iriyama Kyohiko et al., JP (31) 285741, 88; Iriyama Kyohiko et al., CN 89101552). For R2Fe17Nx, easy magnetization axis appears only when R is Sm. Consequently, in the preparation of high performance magnet, the rare earth utilized is mainly Sm that is more costly than Nd or Pr.
In 1990, Yingchang Yang et al. discovered the interstitial atomic effect of nitrogen in R (Fe1xe2x88x92xcex1Mxcex1)12 type intermetallic compound, wherein R is a rare earth element, M is Ti, V, Mo, Nb, Ga, W, Si, Al, or Mn, xcex1 a is from 0.08 to 0.27. The method comprises smelting a master alloy with the above composition, heat treating under nitrogen atmosphere at 350xc2x0 C. to 600xc2x0 C., to form a R(Fe1xe2x88x92xcex1Mxcex1)12Nx interstitial type nitride, such as NdTiFe11Nx. The result of the neutron diffraction study shows that nitrogen atoms have entered the 2b interstitial sites of ThMn12 type crystal structure. Interstitial atoms enhance Fe-Fe exchange, thereby raise the Curie temperature by 200xc2x0 C., modify 3d electron band structure of Fe, thus the magnetic moment of Fe is increased by 10-20%. Most importantly, interstitial atoms adjust the crystal field interactions of rare earth sites in crystals. Upon accomplishment of the nitrogenation, easy magnetization axis appear in the 1:12 type nitride of Pr, Nd, Tb, Dy, and Ho, which have very strong magneto-crystalline anisotropy fields. Therefore, R(Fe1xe2x88x92xcex1Mxcex1)12Nx, particularly Nd(Fe1xe2x88x92xcex1Mxcex1)Nx, has intrinsic magnetic properties comparable to that of Nd2Fe14B, which can be used, besides Nd2Fe14B, as a rare earth permanent-magnetic material based on Nd instead of Sm. (see, for example, CN ZL90109166.9; Yingchang Yang et al., New Potential Hard Materials xe2x80x94Nd(Fe,Ti) 12Nx, Solid State Communications, 78(1991)317; Neutron Diffraction Study of the Nitrides YTiFe11Nx, Solid State Communications, 78(1991)313; and Yingchang Yang et al., Magnetocrystalline Anisotropy of YTiFe11Nx, Applied Physics Letters, 58(1991)2042. Since the publication of these results obtained by Yingchang Yang, there has been disclosed some other patent applications in this field, for example, U.S. Pat. No. 5,403,407 of G. C. Hadjipanayis et al. in 1992. In Hadjipanayis""s patent, an alloy with a composition of RxFeyxe2x88x92wCOwMzLxcex1 is employed, wherein R is a rare earth, M is Cr, Mo, Ti, or V, L is C or N, x is an atomic percent from 5 to 20, y is an atomic percent from 65 to 85, w is an atomic percent of about 20, z is an atomic percent from 6 to 20, and xcex1 is an atomic percent from 4 to 15. In this alloy, it is necessary to add 10-20 atomic percent of cobalt. After smelting of alloy, amorphous non-crystal magnetic material is formed by utilizing a high energy ball-mill mechanical alloying method, and a magnetic powder having a coercivity of 160-640 KA/m (2-8 kOe) is obtained by controlling the crystallization temperature. However, the magnetic powder thus obtained is isotropic, with very low remanence (Br), which is 0.3-0.4 T (3-4 KG), and very low maximum magnetic energy product ((BH)max), which is 8-16 KJ/m3(1-2 MGOe). This does not meet the requirement of practical application. As is well known, the parameters used to denote the performance of a permanent magnetic material include remanence Br, coercivity iHc and bHc, and maximum magnetic energy product (BH)max. In these parameters, the maximum magnetic energy product is an overall indication of permanent magnetism, which represents the overall performance of magnet. In the above said patents, only the intrinsic magnetism of the material, such as saturation magnetization intensity (Ms), Curie temperature (Tc) and anisotropy field of magnetic moment (Ha) are dealt with, while the fundamental performance of permanent magnet are not. In other words, there is not disclosed a method to achieve higher remanence (Br) and higher maximum magnetic energy product ((BH)max). Each of remanence (Br), coercivity (iHc and bHc) and maximum magnetic energy product ((BH)max), which represents the performance of permanent magnetic materials, is structure sensitive. Theoretically, these parameters depend on the structure of magnetic domain and the process of demagnetization. Technically, these parameters depend on the microstructure of the material and the process of its production. This is a very special and very complicated problem that needs to be solved. It is just for this reason that such a category of materials has not been put into practical use since the discovery of the interstitial atomic effect in 1:12 type alloy by Yingchang Yang et al. a decade ago.
It is an object of this invention to provide a multielement rare earth-iron interstitial type permanent magnetic material having a ThMn12 crystal structure. The permanent magnetic material of the present invention has high remanence, high coercivity and high magnetic energy product. There is also provided a process for producing the permanent magnetic material of the present invention.
To this object, the composition of the 1:12 type nitride master alloy is modified based on the result obtained in the study of the magnetic domain structure and the magnetization reversal mechanism of 1:12 type nitride. It is expanded to a multielement alloy, which is featured in that an easily pulverizable alloy with better single-phase property can be produced. This is fundamental for producing high performance magnets. On the other hand, by using the process of the present invention, the activity of alloy is enhanced, the temperature of gas-solid phase reaction is lowered, and complete nitrogenation is ensured. Thereby the magnetism of the material is greatly enhanced, the content of rare earth metal is lowered, and the need to dope with expensive metals such as cobalt is eliminated. By using the process of the present invention, an anisotropic magnetic powder and a magnet having high remanence, high coercivity, and high magnetic energy product can be produced.
Specifically, there is provided a multielement rare earth-iron interstitial permanent magnetic material represent by the following formula:
(R1xe2x88x92xcex1Rxe2x80x2xcex1)x(Mo1xe2x88x92xcex2Mxcex2)yFe100xe2x88x92xxe2x88x92yxe2x88x92zlz
wherein, R is a light rare earth element selected from the group consisting of Pr, Nd, Prxe2x80x94Nd concentrated material and mixtures of Pr and Nd of any composition; Rxe2x80x2 is a heavy rare earth element selected from the group consisting of Gd, Tb, Dy, Ho, Er, Y and a mixture of thereof; xcex1 is from 0.01 to 0.14; x is an atomic percent from 4 to 15; M is an element of IIIA, IVA, IVB, VB, VIB and VIIB families in the periodic table selected from the group consisting of B, Ti, V, Cr, Mn, W, Si, Al, Ga, Nb, Ta, Sr, Zr, and mixtures of thereof; xcex2 is from 0.01 to 0.98; y is an atomic percent from 3 to 20; l is an element occupying the above mentioned interstitial site of the crystal selected from the first and the second periodic groups consisting of H, C, N, F, and mixtures thereof; z is an atomic percent from 5 to 20.
Examples of the permanent magnetic material represented by formula (R1xe2x88x92xcex1Rxe2x80x2xcex1)x(Mo1xe2x88x92xcex2Mxcex2)yFe100xe2x88x92xxe2x88x92yxe2x88x92zlz include:
(Pr0.9Tb0.1)7.0(Mo0.9Nb0.1)7.4Fe77.1N8.5;
(Pr0.9Tb0.1)6.8(Mo0.8Nb0.2)10.0Fe72.9N10.3;
xe2x80x83(Pro0.9Tb0.1)6.8(Mo0.7Nb0.3)10.0Fe72.9N10.3;
(Pr0.92Tb0.08)6.6(MO0.1Ti0.9)6.5Fe73.4N13.5;
(Pr0.92Tb0.08)6.6(Mo0.2Ti0.8)6.5Fe73.4N13.5;
(Pr0.92Tb0.08)6.6(Mo0.3Ti0.7)6.5Fe73.4N3.5;
(Pr0.95Tb0.05)6.5(Mo0.1V0.9)Fe68.3N16.2;
(Pr0.95Tb0.05)6.5(Mo0.2V0.8)9.0Fe68.3N16.2;
(Pr0.95Tb0.05)6.5(Mo0.3V0.7)9.0Fe68.3N16.2,
wherein, Pr may be substituted with Nd, a Prxe2x80x94Nd concentrated material, or a mixture of Pr and Nd; Tb may be substituted with Gd, Dy, Ho, Er, Y, or a mixture thereof; Nb, Ti, V and the like may be substituted with B, Ti, V, Cr, Sr, Mn, W, Si, Al, Nb, Ta, Zr or a mixture of one or more of them.
The light rare earth used in the present invention is preferably Pr, Nd, a mixture of Pr and Nd, or a Prxe2x80x94Nd concentrated material. In the 1:12 type nitrides, Pr and Nd have strong easy axial magnetocrystalline anisotropy, which generate high coercivity. Furthermore, the light rare earth element, Pr and Nd, couple ferromagnetically with Fe, thereby having high saturation magnetization intensity, which is fundamental for producing materials having high remanence and high magnetic energy product. It was found that, for the purpose of producing high performance magnet, it is essential to include a suitable amount of at least a heavy rare earth element such as Gd, Tb, Dy, Ho, Er or the like in the alloy. Only in this way, high performance and high temperature stability of the magnet thus produced can be ensured. In addition, the atomic percent x is preferably from 6 to 10.
As is well known, in order to prepare 1:12 phase based on rare earth-iron, it is necessary to add a suitable amount of a third element M. However, it was discovered by the inventors of the present invention that it is impossible to obtain a high performance magnet by utilizing a R(Fe, M)12 type master alloy comprising only a single third element. In a study aimed to enhance the gas-solid phase reaction, to significantly improve the magnetism of the material, and to facilitate the grinding of the crystal particles, it was further discovered that the third element M is required to be combined with Mo, wherein, M is B, Nb, Ti, V, Cr, Mn, Al, Ga, Si, Sr, Ta, W, or Zr or a mixture of two or more of them. In other words, it is indispensable to include Mo and another third element M in the material so as to produce a high performance magnet of 1:12 type nitride, wherein, M is as defined above. In case that the third element is mainly Mo, xcex2 is preferably 0.01-0.40, and in case that the third element is mainly M, xcex2 is preferably 0.80-0.98. The atomic percent y is preferably 6-12. The results of comparative experiments are shown in Examples 1-12.
The process for producing the magnet of the present invention includes the following steps:
(1) When l is H, N or F, a master alloy is prepared by using R, Rxe2x80x2, Fe, Mo, and M according to the formula (R1xe2x88x92xcex1Rxe2x80x2xcex1)x(Mo1xe2x88x92xcex2Mxcex2)yFe100xe2x88x92xxe2x88x92yxe2x88x92z; when l is C, a master alloy is prepared by using C and the metals of R, Rxe2x80x2, Fe, Mo, and M according to the formula (R1xcex1Rxe2x80x2xcex1)x(Mo1xe2x88x92xcex2Mxcex2)yFe100xe2x88x92xxe2x88x92yxe2x88x92zlz. The alloy thus produced is featured in having a ThMn12 tetragonal crystal structure, thus is referred to as 1:12 type compound. The multielement alloy of the present invention is apt to form uniform 1:12 phase. FIG. 1 and FIG. 2 show the X-ray diffraction pattern of thus obtained Nd7.2Dy0.5V11.0Mo0.5Fe0.8 and Pr6.6Dy0.4Mo9.50Ti0.5Fe76C7, respectively. As can be seen from the figures, they are single 1:12 phase. Further, as can be seen from the thermal magnetic curve of FIG. 3, no xcex1xe2x88x92Fe is included.
(2) The master alloy obtained in step (1) is treated in the hydrogen atmosphere at 200-400xc2x0 C. for 2-4 hours, and a powder with the particle size in microns is formed. It was found that hydrogen holds 2b interstitial sites in the alloy, which is similar to nitrogen. Thus, hydrogen has an interstitial atomic effect for improving magnetism similar to that of nitrogen, as is shown in Example 14. Hydrogen treatment is a pretreatment for nitrogenation. As the result of hydrogen treatment, the activity of the material is enhanced, thereby, the particle size of the nitrided powder is enlarged, and the temperature and the time of nitrogenation are reduced. This is one of the measures to avoid oxidation and to ensure complete nitrogenation of the materials, thereby enhancing permanent magnetic properties. Especially, when y is selected from the lower part of the above said range, the performance of the hard magnetic properties varies significantly depending upon whether or not hydrogenation treatment is carried out, as is shown in Example 15.
(3) Gas-solid phase reaction of the above treated powder is allowed to proceed in a corresponding atmosphere l at given temperatures. For example, when l is N, heat treatment is carried out in nitrogen atmosphere of 1-10 atmospheric pressure at 300-650xc2x0 C. for 1-20 hours. As the result of the gas-solid phase reaction, a nitride having the composition of (R1xe2x88x92xcex1Rxe2x80x2xcex1)x(Mo1xe2x88x92xcex2Mxcex2) yFe100xe2x88x92xxe2x88x92yxe2x88x92zNz is formed. The nitride thus obtained features in a ThMn12tetragonal crystal structure, accordingly is referred to as 1:12 type nitride. The multielement alloy and the process provided by the present invention facilitate the proceeding of the gas-solid reaction. Complete nitrogenation can be achieved under single-phase conditions, namely under the conditions of absence of oxide and xcex1xe2x88x92Fe. Nitrogen content of the magnet is up to 5-20 at %. Compared with 1:12 type master alloy, the Curie temperature and the saturation magnetization of 1:12 master alloy are significantly increased, and the magnetocrystalline anisotropy of rare earth ions is changed after nitrogen absorption, particularly, the easy magnetization direction is c axis from OK to Curie temperature for 1:12 nitride of Pr, Nd, Tb, Dy, and Ho. When I is F, heat treatment is carried out at a temperature of 200-500xc2x0 C. in a fluorine atmosphere of 1-4 atmospheric pressure for 1-2 hours, then a corresponding fluoride of the invention is obtained. The result of the band structure analysis shows that, the fluorine has an optimum interstitial atomic effect. In fluorides, the increment of magnetic moment of iron atoms is greater than that in nitrides or carbides.
(4) The 1:12 type material processed through above step 2) and/or step 3) is pulverized into a powder with the particle size of 1-10 xcexcm by using a jet mill or ball mill. An anisotropic high performance magnetic powder is formed, which is featured in that the maximum magnetic energy product is above 160 KJ/m3 (20MGOe).
(5) A protective coat can be formed on the surface of magnetic powder by using 1:12 type magnetic powder through complex reaction, such as, the magnetic powder obtained from step (3) is milled in the solution of citric acid, ammonium acetate or potassium thiocyanate. Oxidation resistance is enhanced through the metal-complex reaction.
(6) A binder is added into the coated magnetism powder, then oriented through press molding in a magnetic field. A high performance anisotropic bonded magnet is obtained upon solidification.
In another embodiment of the present invention, the 1-10 xcexcm magnetic powders are directly mixed with a polymer or rubber. Then a bonded magnet is formed through injection molding in a magnetic field. Alternatively, it is mixed with a low-melting-point metal, such as Zn, Sn and the like, or an alloy thereof, then pulverized into a powder with the particle size of 1-10 xcexcm, oriented in a magnetic field, press molded, and an anisotropic sintered magnet is obtained upon sintering.
In the process of the present invention, when l is C, step (2) and (3) are not needed. The process of the invention is featured in that C enters an interstitial site through direct melting, instead of through gas-solid reaction. The magnetic powders of carbide according to the present invention have the advantages of good temperature stability. When I is H, the process can be carried out to step (4) directly without passing step (3).
In addition, a high performance magnet can be produced through other methods by utilizing the multielement alloy of the present invention. Examples of these methods include mechanical alloying, which comprises the steps of: when I is N, (1) the metallic powders of R, Rxe2x80x2, Fe, Mo, M and the like are subjected to high energy milling in argon atmosphere for 2-4 hours with the composition of (R1xe2x88x92xcex1Rxe2x80x2xcex1)x(Mo1xe2x88x92xcex2Mxcex2)yFe100xe2x88x92xxe2x88x92y to obtain the resulted amorphous metal powders; (2) carrying out crystallization treatment in argon atmosphere at 700-950xc2x0 C., and keeping at this temperature for 0.5-2 hours; (3) carrying out gas-solid reaction in the atmosphere of interstitial atom, for example, carrying out nitrogen treatment at 400-600xc2x0 C. for 2-4 hours, then a high performance magnetic powder is obtained. In this method, when l is C, step (1) is performed by preparing a corresponding powder according to formula (R1xe2x88x92xcex1Rxe2x80x2xcex1)x(Mo1xe2x88x92xcex2Mxcex2)yFe100xe2x88x92xxe2x88x92yCz, conducting high energy milling in argon atmosphere for 2-4 hours, thus, a amorphous powder is formed; step (2) is performed as mentioned above, and step (3) is omitted; finally, a high performance magnetic powder is formed. These method also include melt spinning method, which comprise the steps of: when I is N, (1) melting an alloy with the composition of (R1xe2x88x92xcex1Rxe2x80x2xcex1)x(Mo1xe2x88x92xcex2Mxcex2)yFe100xe2x88x92xxe2x88x92y; (2) cooling in vacuum at the rate of 30-50 m/second; (3) carrying out crystallization treatment in argon atmosphere at 700-950xc2x0 C., and keeping at this temperature for 0.5-2 hours; (4) carrying out gas-solid reaction in the atmosphere of interstitial atom, for example, carrying out nitrogen treatment at 400-600xc2x0 C. for 2-4 hours, then a high performance magnetic powder is obtained. In this method, when l is C, step (1) is performed by melting an alloy according to formula (R1xe2x88x92xcex1Rxe2x80x2xcex1)x(Mo1xe2x88x92xcex2Mxcex2)yFe100xe2x88x92xxe2x88x92yCz; steps (2) and (3) are performed as mentioned above, and step (4) is omitted; finally, a high performance magnetic powder is formed.
In the process of the present invention, after hydrogenation treatment, evacuated dehydrogenation treatment at 500-600xc2x0 C. may be performed.
By using the magnetic powder of the present invention, compressed, injected and extruded bonded magnet can be produced by adding a thermosetting binder, or a calendered bonded magnet can be produced by adding a thermoplastic binder. In particular, compressed and injected anisotropic bonded magnets can be produced by taking form in a magnetic field. Further, mixing the magnetic powder of the present invention and a ferrite magnetic powder can produce a unique composite magnet. Since the particle size of them are comparable, a uniform calendered, injected or compressed bonded magnet can be produced. As the magnetic powder of the present invention has high remanence while ferrite has a positive temperature coefficient of coercivity, thus it is available to produce composite magnets with high magnetic performance and good thermal stability at a low cost.
The binders used in the present invention includes polyolefin polymers, such as polyethylene, polypropylene, polyvinyl chloride, nylon and the like; polyester polymers, such as polyether, polyurethane, polycarbonate and the like; aromatic polyester resins, such as epoxy resin, phenolic resin, pollopas and the like; natural or synthetic rubbers, such as natural rubber, butadiene rubber, duprene rubber, silicon rubber and the like.
To assist in understanding the present invention, the following Examples are included which describes the results of a series of experiments. The experiments relating to this invention should not, of course, be construed as specifically limiting the invention.